This invention relates to a method for the manufacture of an aqueous suspension of a solid organic peroxide, and more particularly to a method for safe manufacture of an aqueous suspension of a solid organic peroxide without requiring any special machine for dispersion.
Organic peroxides decompose with cleavage and liberate free radicals. Because of this behavior, organic peroxides are capable of selectively effecting radical polymerization or cross-linking reaction. A concrete example of the former reaction is the (co)polymerization of ethylenically unsaturated monomers such as, for example, vinyl chloride. As a concrete examples of the latter reaction, the cross-linking reaction of polyethylene and ethylene-vinyl acetate copolymer is well known. At normal room temperatures, some of organic peroxides are liquid and others are solid. Examples of organic peroxides which are liquid at normal room temperatures are di-n-propyl peroxydicarbonate and di-2-ethylhexyl peroxydicarbonate. Among the organic peroxides which are used in polymerization ethylenically unsaturated monomers at temperatures not exceeding 60.degree. C., e.g. vinyl chloride, many suffer from inferior storageability. In contrast, those organic peroxides such as, for example, di-cetyl peroxydicarbonate and di-decanoyl peroxide which are solid at normal room temperatures are generally known to excel in storageability and to undergo only slight degradation during protracted storage. On the other hand, such solid organic peroxides have a disadvantage that they are difficult to handle because of their lack of fluidity and they dissolve slowly and are not readily dispersed because of their large particle diameter. Thus, the last few years have seen active work toward the development of aqueous suspensions of solid organic peroxides which retain the merits of solid organic peroxides intact, exhibit high fluidity and solubility, flow smoothly enough to permit conveyance by pumps, and excel in stability. For example Japanese patent Disclosure Sho No. 51(1976)-125302 discloses a method which comprises homogenizing in a three-roll mill a mixture consisting of water, a protective colloid, a surface active agent, and a solid organic peroxide; Japanese patent Disclosure Sho No. 54(1979)-25992 discloses a method which comprises finely dividing a mixture consisting of water, a solid organic peroxide, and a protective colloid in a pulverizer such as a ball mill, hammer mill, or grinder; and Japanese patent Disclosure Sho No. 56(1981)-110702 discloses a method which comprises dispersing a mixture consisting of a protective colloid, a surface active agent, water, and a solid organic peroxide in an Ultraturrax and further homogenizing the resultant dispersion in an ultrasonic homogenizer. Further, Japanese patent Disclosure Sho No. 57(1982)-501085 discloses a method which comprises dispersing a mixture consisting of water, an organic peroxide, a surface active agent, and a protective colloid by passing the mixture into a colloid mill; the specification of West German Pat. No. 2,634,131 discloses a method which comprises dispersing in an ultrasonic homogenizer a mixture consisting of a solid organic peroxide, a protective colloid, and water and the specification of U.S. Pat. No. 3,849,148 discloses a method which comprises mixing a non-ionic surface active agent with a solid organic peroxide at temperature in the range of 60.degree. C. to 66.degree. C., then effecting phase inversion on the resultant emulsion, and subsequently adding water thereto until the temperature of the system falls to the ambient temperature. These methods disclosed by the various publications each entails some demerits. For example, most of the aforementioned methods necessitate special dispersing machines possessing strong shearing stress. Use of such dispersing machines has entails the danger that during pulverization and dispersion, solid organic peroxides will be heated and consequently degraded. The method disclosed by the aforementioned U.S. patent specification has no need for any special device but entails the following demerits. Firstly, since a non-ionic surface active agent is mixed with a solid organic peroxide at a temperature in the range of 60.degree. C. to 66.degree. C., if the organic peroxide happens to be one which, like di-cetyl peroxydicarbonate or di-decanoyl peroxide, lacks thermal stability, then this method cannot be effectively applied to this organic peroxide because the organic peroxide is liable to be explosively decomposed. Secondly, the aqueous suspension obtained by this method is susceptible of sedimentation or creaming of particles.
As described above, when aqueous suspensions of organic peroxides are produced by such conventional methods, the dispersed particles are not sufficiently fine and, consequently, undergo sedimentation or the like. Thus, the conventional methods have been incapable of producing stable aqueous suspensions.